1. Field of the Invention
The present invention relates generally to an anti-skid brake control system for automotive vehicle, which controls braking pressure in order to maintain wheel slippage at an optimum level for optimal vehicular braking performance. More specifically, the invention relates to a technology for detecting road surface friction level for precise control of skid control cycle and adapting skid control schedule to the road friction level.
2. Description of the Background Art
Recently, various constructions and operations of anti-skid vehicular brake control systems have been proposed for improving deceleration performance of an automotive vehicle for assure safety in braking. In the anti-skid brake control system, skid control cycle, in which braking pressure is controlled or adjusted so as to avoid excessive wheel slippage due to locking or skiding of wheel, is initiated in response to a wheel deceleration (negative value of acceleration). During anti-skid brake control, braking pressure is adjusted over one or more skid control cycles for maintaining braking pressure in the vicinity of lock pressure to maintain wheel slippage at an optimal level, i.e. 10 to 20%.
One of the typical process of anti-skid brake control had been disclosed in Japanese Patent First (unexamined) Publication (Tokkai) Showa 60-261769. In the disclosed system, a vehicle body speed representative data is derived on the basis of a wheel speed indicative data. The system derives an approximated road friction indicative data on the basis of gradient of deceleration of wheel speed. Such technology is practically useful for second and subsequent skid cycles of anti-skid control. However, for the first skid control cycle, the gradient data cannot be obtained to make it impossible to derive the road friction indicative data. Therefore, it is usual technology to use a predetermined gradient value for approximating the deceleration of the vehicle body for the first skid control cycle. For this, the predetermined gradient value has to be set in view of a certain road friction level. Selection of the road friction level is difficult matter. Namely, when the gradient value is set in view of relatively low road friction level, braking pressure reduction is taken placed at too early timing to optimize braking characteristics. On the other hand, when the gradient value is set in view of relatively high friction level, excessive wheel slippage may occur at low friction road to cause expansion of the braking distance.